1. Field of the Invention
The invention relates to a steering system for a vehicle and, more particularly, to a steer-by-wire steering system.
2. Description of Related Art
In recent years, various systems relating to steering of an automobile, that is, so-called steer-by-wire systems, have been developed for practical use. An existing vehicle is generally configured such that a steering wheel and steered wheels are mechanically coupled to each other via a mechanism, such as a rack-and-pinion. However, in a steer-by-wire system, there is no mechanical connection between these components. In the steer-by-wire system, an input from a driver, such as a torque and a steering angle, is detected by a sensor, a steered angle appropriate for a travel state of a vehicle is obtained together with information from another vehicle sensor, the steered angle command value is transmitted to a steering actuator, and then wheels are actually steered.
In some of steering systems that employ the steer-by-wire system, a backup clutch is provided between the steering wheel and the steered wheels. That is, even when the steer-by-wire system fails, the backup clutch mechanically connects the steering wheel to the steered wheels, thus making it possible to ensure steering performance.
In the steering system that employs such a steer-by-wire system, it is important that the steering wheel is reliably connected to the steered wheels by the backup clutch at the time of a failure of the system. Therefore, there is known an existing technique for, for example, when the power of a system is turned on, checking whether mechanical connection of a steering wheel with steered wheels by a backup clutch is normally performed by determining whether there occurs movement in the steered wheels and the steering wheel while a connection command is output to the backup clutch to drive a turning actuator and a steering reaction actuator (for example, see Japanese Patent Application Publication No. 2009-179185 (JP 2009-179185 A)).
However, in the system described in JP 2009-179185 A, there is a possibility that the backup clutch is not appropriately disengaged after checking the operation of the backup clutch and then the steering wheel and the steered wheels remain mechanically connected to each other. Thus, there is a possibility that steer-by-wire control is started while the steering wheel and the steered wheels are mechanically coupled to each other.